1. Field of the Invention
This invention pertains to the field of separating and purifying at least one gas component of a feed gas by a pressure swing adsorption (PSA) process. More particularly, the present invention relates to an integrated pressure swing adsorption/membrane separation process for the separation and purification of at least one gas component of a feed gas in which purge effluent from the PSA system is passed through a membrane separation system and the resulting non permeate is then utilized as a displacement gas or a copurge in the PSA system.
2. Discussion of Related Art
The PSA process is a well known means for separating and purifying a less readily adsorbable gas component contained in a feed gas mixture from a more readily adsorbable second component.
Pressure swing adsorption systems generally involve passage of the feed gas mixture through equipment comprising two or more adsorbers containing beds of molecular sieves or other adsorbents which selectively adsorb the heavier components of the gas mixture. The adsorbers are arranged to operate in sequence with suitable lines, valves, timers and the like so there are established an adsorption period during which the heavier components of the feed gas mixture are adsorbed on the molecular sieve or other adsorbent and a regeneration period during which the heavier components are desorbed and purged from the adsorbent to regenerate it for reuse.
Such selective adsorption commonly occurs in the adsorption beds at an upper adsorption pressure, with the more selectively adsorbable component thereafter being desorbed by pressure reduction to lower desorption pressure. The beds can be purged at such lower pressures for further feed gas purification.
Such PSA processing is disclosed in U.S. Pat. No. 3,430,418 to Wagner and in U.S. Pat. No. 3,986,849 to Fuderer et al., wherein cycles based on the use of multi-bed systems are described in detail. As is generally known and described in these patents, the contents of which are incorporated herein by reference as if set out in full, the PSA process is generally carried out in a sequential processing cycle that includes each bed of the PSA system. Such cycles are commonly based on the release of void space gas from the product end of each bed in one or more cocurrent depressurization steps upon completion of the adsorption step. In these cycles, the released gas typically is employed for pressure equalization and for subsequent purge steps. The bed is thereafter countercurrently depressurized and/or purged to desorb the more selectively adsorbed component of the gas mixture from the adsorbent and to remove such gas from the feed end of the bed prior to the repressurization thereof to the adsorption pressure.
PSA processes were first used for gas separations in which only one of the key components was recovered at high purity. For example, from 100 moles feed gas containing 80 moles hydrogen and 20 moles carbon monoxide, the process of Wagner, U.S. Pat. No. 3,430,418, could separate 60 moles of hydrogen at 99.999% purity, but no pure carbon monoxide could be recovered; 20 moles of carbon monoxide and 20 moles of hydrogen remained mixed at 50% purity each. A complete separation could not be made. Only the less adsorbable, light component was recovered at high purity.
For the recovery of a pure, more strongly adsorbed heavy component, an additional step was necessary, namely, rinsing of the bed with a heavy component to displace the light component from the bed prior to depressurization. This rinsing step is described in several earlier patents. The problems with these processes are the following: (a) if the rinsing is complete and the light component is completely displaced from the bed, pure heavy component can be obtained, but the adsorption front of the heavy component breaks through to the light component and the latter cannot be recovered at high purity; (b) if the displacement of the light component is incomplete, the typical concentration profile of the heavy component in the bed as indicated at FIG. 2 of the present application is obtained, and if such bed is depressurized countercurrently to recover the heavy key component at the feed end, the light component still present in the bed reaches the feed end very rapidly and the purity of the heavy component drops. It is therefore not practical with the prior art processes to obtain both key components at high purity in a single PSA unit.
Such complete separations can be obtained, however, by two separate pressure swing adsorption processing units wherein each unit includes several fixed beds. From a feed gas containing, for example, hydrogen and carbon monoxide (CO), the first unit recovers pure hydrogen and a carbon monoxide rich gas containing 70 percent carbon monoxide. This gas mixture is compressed and passed through a second PSA unit which recovers pure carbon monoxide and a hydrogen rich gas. The hydrogen rich gas can be added as feed gas to the first PSA unit and then the cycle is repeated. The combination of the two independent PSA units can make an excellent separation at very high flexibility. For example, from a gas mixture with two components this system can recover more than 99.8 percent of the adsorbable "light" component such as hydrogen at a purity of 99.999 percent and also recover essentially 100 percent of the more readily adsorbed, heavy component, such as carbon monoxide, at a purity higher than 99.5 percent.
A PSA process suitable for the recovery of both the less and more readily adsorbable components is described in British patent No. 1,536,995 to Benkmann. The process is based on two beds in series cycle as shown in FIG. 2 of Benkmann. The feed is introduced to the lower bed which retains the more readily adsorbable component. The feed step is followed by a copurge step in which the less readily adsorbable or light component is displaced in the lower bed by a recycled stream of heavy components, so that the lower bed at the end of the step contains only the heavy component. At this moment, the connection between the upper and lower beds is interrupted by an automatic valve and the heavy product is recovered from the lower bed by (countercurrent) depressurization. The upper bed is, in the meantime, also depressurized and purged to remove all of the heavy component. The step sequence of the upper and lower bed are interlocked and cannot be run with independent cycles. The flexibility of this system is therefore reduced while the complexity is increase. Problems with this system are: a set of two beds in series is needed; if process conditions such as feed gas composition change, it is not possible to change the volume ratio of the two beds which means lower flexibility; the vessel heads of the two beds contain more void space gas which increases depressurization loss and compressor power; and the pressure drop is also increased.
In copending, commonly assigned U.S. Pat. No. 4,723,966, issued Feb. 9, 1988, the contents of which are incorporated herein by reference as if set out in full, a PSA method is disclosed in which binary separations are achieved in single adsorption beds. Thus, after the adsorption step has proceeded to a point where the bed is sufficiently charged, the gas mixture within the bed is displaced or substituted with a gas stream containing the more readily adsorbable components. After this displacement step, the feed end of the bed contains substantially pure, more readily adsorbable components and the outlet end of the bed contains substantially pure, less adsorbable components. The thusly polarized bed is then depressurized simultaneously from both ends, thereby removing the separated, substantially pure components from their respective ends.
Attempts to purify gas streams employing other means have also been attempted, particularly utilizing semi-permeable membranes. However, such semi-permeable membrane gas separation processes, while able to separate the less permeable component, i.e., the non permeate stream, at relatively high purity, generally have not been capable of providing permeating components at high purity. Indeed, even with two- or three-stage permeation, as illustrated in U.S. Pat. No. 4,264,338, only moderate purity of the permeate stream is obtained in conjunction with costs which are economically unattractive.
Itegration of semi-permeable membrane units with PSA systems have also taken place. Thus, in U.S. Pat. No. 4,229,188 and 4,238,204, a semi-permeable membrane separation unit is utilized to treat purge gas obtained from the regeneration of a selective adsorption bed wherein the permeated light gas is recycled with the feed gas mixture for further treatment in the selective adsorption bed and the non-permeated heavy gas is entirely removed from the system and generally utilized as a fuel gas.
In a more recent application of the use of a semi-permeable membrane in conjunction with a PSA process, as disclosed in U.S. Pat. No. 4,398,926, a feed gas containing a high concentration of impurities is first passed through a separator containing a permeable membrane capable of selectively permeating hydrogen. The separator is used to achieve a bulk separation of the desired hydrogen from the impurities contained in the gas stream. The separated hydrogen is recovered at a reduced pressure and is passed to the pressure swing adsorption which is adapted for operation at the reduced pressure. The non-permeated gas from the separator is recovered essentially at the higher pressure of the gas stream and a portion thereof is throttled to a lower pressure and passed through the pressure swing adsorption system as a co-feed gas.
There accordingly still remains a desire in the art to more effectively and economically utilize semi-permeable membrane separation techniques in conjunction with PSA systems for the purification of gas mixtures.